Mr Brown described the SNP's quest for "fiscal autonomy" - giving the Scottish Parliament more financial powers - as the "economics of separation" that was a "repackaging of their old constitutional fundamentalism".

He said: "Fiscal separation, even under the guise of slashing business taxes, is not SNP generosity to Scottish businesses, but the SNP attempt to use Scottish business as their battering ram for separation."

Billions would be spent on creating the apparatus for a separate Scottish state including new social security systems, inland revenue and customs and excise headquarters.

Mr Brown said the extra costs and spending associated with independence would take Scotland beyond the Maastricht deficit limit of three per cent of gross domestic product.

He said the SNP policies threatened the "newly-won and hard-won economic stability" achieved by Labour, which was crucial to breaking away from the boom-bust cycles of the past.

He added that the SNP was unable to say how interest rates would be set if they won a referendum on independence. "When an independence party has no real clue where it is going on its monetary policy, we are right to fear that divorce from the UK would hurt our economy and undermine the social fabric of the country," he said.

"Given this confusion over a fundamental issue of Nationalist economic policy is as glaring as at any point in the SNP's history; the choice is stark - stability under Labour versus instability under the Nationalists."

But John Swinney, the SNP leader, said: "Gordon Brown may sit for a Scottish constituency but he is no Scottish Chancellor.

"All he has to offer is low growth, low employment and fewer opportunities. His speech is nothing more than a smokescreen to cover his disastrous management of the Scottish economy."